In a desulfurization facility in which SO2 in flue gas is desulfurized by a limestone-gypsum method, a gypsum dewatering device that acquires gypsum by dewatering gypsum slurry generated in a desulfurization process is applied. As a type of the gypsum dewatering device, there is a gypsum dewatering device that uses a belt filter, in which a filter fabric is arranged in a belt-like shape, and a vacuum pump and a decompression mechanism are provided below the belt, to dewater gypsum slurry fed onto the belt by vacuum suction from below the belt, while advancing the filter fabric belt in a horizontal direction. However, the gypsum slurry contains fine impurities, for example, fine particle dust including silica (SiO2), aluminum oxide (AlO3), and unburned carbon. Therefore, if a fine particle dust layer is formed on the uppermost layer on the surface of a gypsum cake on the belt filter, which is formed in a uniform thickness, while dewatering the gypsum slurry, the air permeability is deteriorated at the time of vacuum suction, thereby causing a decrease in the dewatering performance. When the dewatering performance decreases, the adhesive property of the gypsum cake increases, and this causes a problem such that the gypsum cake adheres to the circumference in a transportation process. Consequently, the quality of the gypsum does not meet a criteria (for example, 10 wt %), and the gypsum cannot be sold as a product, and thus the disposal cost for disposing it as industrial waste is required.
Conventionally, therefore, for example, in a belt filter described in Patent Literature 1, comb-like gates are provided in a plurality of stages in a longitudinal direction of the belt filter, whose lower ends extend to an upper layer of a gypsum cake, so that a fine particle layer formed on the surface of the gypsum cake is stirred, thereby improving its dewaterability.